IndexMap

In my project there are several idiomatic uses of JDK collections. One is using Map to uniquely index domain objects by some field. The code is simple:

public class IndexMap extends SafeHashMap {
    public static interface Mapper {
        public Object getKeyFor(final Object value);
    }

    private final Mapper mapper;

    public IndexMap(final Class keyClass, final Class valueClass,
            final Mapper mapper) {
        super(keyClass, valueClass);

        this.mapper = mapper;
    }

    public IndexMap(final Class keyClass, final Class valueClass,
            final Mapper mapper, final Collection values) {
        this(keyClass, valueClass, mapper);

        addAll(values);
    }

    public void add(final Object value) {
        put(mapper.getKeyFor(value), value);
    }

    public void addAll(final Collection values) {
        for (final Iterator it = values.iterator(); it.hasNext();)
            add(it.next());
    }
}

(SafeHashMap is another JDK collection extension. It forbids null keys and values, and requires they be of certain classes.)

Idiomatic use looks like:

new IndexMap(KeyType.class, DomainType.class, new IndexMap.Mapper() {
    public Object getKeyFor(final Object value) {
        return ((DomainType) value).getKey();
    }
}, initialValues);

Which indexes a collection of DomainType domain objects by the key property.

UPDATE: Because of editing several files at once, I suffered a brain fart and mixed IndexMap (the point of this post) with AutoHashMap (another, still interesting collection).